In multicarrier or carrier aggregation (CA) operation, the UE is able to receive and/or transmit data to more than one serving cells. In other words a CA capable UE can be configured to operate with more than one serving cell. The carrier of each serving cell is generally called a component carrier (CC). Said differently, the component carrier (CC) is an individual carrier in a multi-carrier system. The carrier aggregation (CA) is interchangeably referred to as a “multi-carrier system”, “multi-cell operation”, “multi-carrier operation”, “multi-carrier” transmission and/or reception. This means the CA is used for transmission of signaling and data in the uplink and downlink directions. One of the CCs is the primary component carrier (PCC) or simply primary carrier or even anchor carrier. The remaining ones are called secondary component carrier (SCC) or simply secondary carriers or even supplementary carriers. The serving cell is interchangeably called as primary cell (PCell) or primary serving cell (PSC). Similarly, the secondary serving cell is interchangeably called as secondary cell (SCell) or secondary serving cell (SSC).
Generally, the primary or anchor CC carries the essential UE specific signaling. The primary CC (aka PCC or PCell) exists in both uplink and downlink directions in CA. In case there is single UL CC the PCell is on that CC. The network may assign different primary carriers to different UEs operating in the same sector or cell.
In dual connectivity (DC) operation the UE can be served by at least two nodes called master eNB (MeNB) and secondary eNB (SeNB). More generally, in multiple connectivity (aka multi-connectivity) operation the UE can be served by two or more nodes where each node operates or manages one cell group e.g. MeNB, SeNB1, SeNB2 and so on. More specifically, in multi-connectivity each node serves or manages at least secondary serving cells belonging its own cell group. Each cell group contains one or more serving cells. The UE is configured with PCC from both MeNB and SeNB. The PCell from MeNB and SeNB are called as PCell and PSCell respectively. The UE is also configured with one or more SCCs from each of MeNB and SeNB. The corresponding secondary serving cells served by MeNB and SeNB are called SCells. The UE in DC typically has separate TX/RX for each of the connections with MeNB and SeNB. This allows the MeNB and SeNB to independently configure the UE with one or more procedures e.g. radio link monitoring (RLM), DRX cycle etc. on their PCell and PSCell respectively.
In multi-connectivity all cell groups may contain serving cells of the same radio access technology (RAT), for example LTE, or different cell groups may contain serving cells of different RATs.
Dual Connectivity in LTE
E-UTRAN supports Dual Connectivity (DC) operation whereby a multiple Rx/Tx UE in RRC_CONNECTED is configured to utilize radio resources provided by two distinct schedulers, located in two eNBs connected, for example, via a non-ideal backhaul over the X2 interface. eNBs involved in DC for a certain UE may assume two different roles: an eNB may either act as an MN (Master node) or as a SN (Secondary node). In DC, a UE may be connected to one MN and one SN.
FIG. 1 illustrates and example LTE DC user plane, according to certain embodiments. In LTE DC, the radio protocol architecture that a particular bearer uses depends on how the bearer is setup. Three bearer types exist: MCG (Master Cell Group) bearer, SCG (Secondary Cell Group) bearer and split bearers. RRC is located in MN and SRBs (Signaling Radio Bearers) are configured as MCG bearer type and therefore only use the radio resources of the MN.
LTE-NR Dual Connectivity
LTE-NR (New Radio) DC (also referred to as LTE-NR tight interworking) is currently being discussed for rel-15. In this context, the major changes from LTE DC are:                The introduction of split bearer from the SN (known as SCG split bearer)        The introduction of split bearer for RRC        The introduction of a direct RRC from the SN (also referred to as SCG SRB or SRB3)        
FIGS. 2 and 3 show the user plane (UP) and Control Plane (CP) architectures for LTE-NR tight interworking. FIG. 2 illustrates an example LTE-NR tight interworking in the user plane. FIG. 3 illustrates an example LTE-NR tight interworking in the CP. The SN is sometimes referred to as SgNB (where gNB is an NR base station), and the MN as MeNB in case the LTE is the master node and NR is the secondary node. In the other case where NR is the master and LTE is the secondary node, the corresponding terms are SeNB and MgNB
Split RRC messages are mainly used for creating diversity, and the sender can decide to either choose one of the links for scheduling the RRC messages, or it can duplicate the message over both links. In the downlink, the path switching between the MCG or SCG legs or duplication on both is left to network implementation. On the other hand, for the UL, the network configures the UE to use the MCG, SCG or both legs. The terms “leg” and “path” are used interchangeably throughout this document.                DC (Dual Connectivity): LTE DC (both MN and SN employ LTE)        EN-DC (E-UTRAN, New Radio DC): LTE-NR dual connectivity where LTE is the master and NR is the secondary        NE-DC (New Radio, E-UTRAN DC): LTE-NR dual connectivity where NR is the master and LTE is the secondary        NR-DC (or NR-NR DC): both MN and SN employ NR        MR-DC (multi-RAT DC): a generic term to describe where the MN and SN employ different RATs (EN-DC and NE-DC are two different example cases of MR-DC)        
In E-UTRAN-NR dual connectivity, the master cell group contains at least E-UTRA PCell while secondary cell group contains at least NR PSCell. In this example, master CG and secondary CG are managed by eNB and gNB respectively. In NR-E-UTRAN dual connectivity, the master cell group contains at least NR PCell while secondary cell group contains at least LTE PSCell. In this example, master CG and secondary CG are managed by gNB and eNB respectively.
Measurement Gaps in LTE
Inter-frequency measurements in LTE are conducted during periodic inter-frequency measurement gaps which are configured in such a way that each gap starts at an SFN and subframe meeting the following conditions:                SFN mod T=FLOOR(gapOffset/10);        subframe=gapOffset mod 10;        
with T=MGRP/10, where MGRP stands for “measurement gap repetition period”. E-UTRAN must provide a single measurement gap pattern with constant gap duration for concurrent monitoring of all frequency layers and RATs. Two configurations are supported by the UE, with MGRP of 40 and 80 ms, both with the measurement gap length (MGL) of 6 ms. In practice, due to the switching time, this leaves less than 6 but at least 5 full subframes for measurements within each such measurement gap. Shorter MGL has been recently also standardized in LTE.
In LTE, measurement gaps are configured by the network to enable measurements on the other LTE frequencies and/or other RATs. The gap configuration is signalled to the UE over RRC protocol as part of the measurement configuration. The gaps are common (i.e., shared by) for all frequencies, but the UE can measure only one frequency at a time within each gap.
In the RRC specifications, measurement gaps and procedures related to it may be described, for example, in TS 36.331 section 5.5.
5.5 Measurements
5.5.1 Introduction
The UE reports measurement information in accordance with the measurement configuration as provided by E-UTRAN. E-UTRAN provides the measurement configuration applicable for a UE in RRC_CONNECTED by means of dedicated signalling, i.e. using the RRCConnectionReconfiguration or RRCConnectionResume message.. . .                5. Measurement gaps: Periods that the UE may use to perform measurements, i.e. no (UL, DL) transmissions are scheduled.        5.5.2 Measurement configuration        5.5.2.1 General        . . .        1>if the received measConfig includes the measGapConfig:                    2>perform the measurement gap configuration procedure as specified in 5.5.2.9;                            5.5.2.9 Measurement gap configuration                                                The UE shall:                    1>if measGapConfig is set to setup:                            2>if a measurement gap configuration is already setup, release the measurement gap configuration;                2>setup the measurement gap configuration indicated by the measGapConfig in accordance with the received gapOffset, i.e., the first subframe of each gap occurs at an SFN and subframe meeting the following condition (SFN and subframe of MCG cells):                                    SFN mod T=FLOOR(gapOffset/10);                    subframe=gapOffset mod 10;                                                with T=MGRP/10 as defined in TS 36.133 [16];                                    NOTE: The UE applies a single gap, which timing is relative to the MCG cells, even when configured with DC.                                                                    1>else:                            2>release the measurement gap configuration;. . .6.3.5 Measurement information elements. . .                                                
MeasConfig
The IE MeasConfig specifies measurements to be performed by the UE, and covers intra-frequency, inter-frequency and inter-RAT mobility as well as configuration of measurement gaps.